MR markers may be subdivided into passive and active MR markers. Passive markers may be detected in the MR image by their special contrast if they are situated in the imaging plane. The contrast is caused either by signal extinguishing (e.g., negative contrast) or signal amplification (e.g., positive contrast), as in the case of the inductively coupled coil. Signal extinctions are, for example, caused by susceptibility markers made from dysprosium oxide (Dy2O3). The susceptibility differences between the markers and the surrounding tissue lead to field gradients that cause a more rapid dephasing of the magnetization, and thus a signal reduction. It is also possible, by relocating the magnetization, to achieve a signal amplification by simultaneously suppressing the background signal (e.g., a so-called white marker). There are also systems that cause switchable signal changes and may thus, for example, be detected by change in the signal. Systems with direct signal readout, (e.g., small receive coils), are denoted as active marker systems.
MR markers are used today in magnetic resonance imaging to detect and/or to mark the current position of the marker in an examination volume. MR markers are, furthermore, used to measure dynamic and static properties of the electromagnetic fields (e.g., B1, B0, gradient fields) of the magnetic resonance imaging scanner.
An active MR marker may include a vessel (e.g., a tube) with a marker medium, for example, a small quantity of water (optionally doped with NiSO4 or similar salt to reduce the T1/T2 times). The tube is surrounded by a coil that detects the MR signals that come from the marker medium (e.g., water) of the tube. The MR signals are, for example, used to measure fluctuations in the B0 field or to measure gradient effects (e.g., eddy currents of the gradient coils). In this case, the TX field exciting the marker medium is, for example, produced by the same coil of the magnetic resonance imaging scanner as the TX field. The same coil may also be used for the MR imaging (e.g., the so-called “body coil”, alternatively, or in addition, also local coils). The signals received by the coil, which surrounds the tube, are evaluated. However, the signals emitted by the marker medium are also received by the receive coil of the magnetic resonance imaging scanner for imaging (e.g., a body coil or local coil), and may lead to convolutions in the MR imaging.
The problems associated with the use of previously known MR markers reside in the fact that the MR markers produce MR signals and may therefore produce pixels outside an examination object during MR imaging. In order also to image the pixels during a measurement, the field of view (FOV) is enlarged. This leads to extension of the measurement time, or a risk of convolutions that reduce the diagnostic value of the MR image produced, and thereby renders misdiagnosis possible through convolution of the marker signal.